1. Field of the Invention
This invention relates to an optical connector and, in particular, to an optical connector that is capable of easy connecting a holey fiber at a setting site thereof.
2. Description of the Related Art
At present, in order to connect optical fibers, a physical connecting method is widely used that is conducted by butting against each other end faces of the optical fibers.
The physical connecting method includes a connecting method using a mechanical splice and a connecting method using an optical connector.
At the connection work site of the optical fibers, for example, an optical connector is used that is composed of a ferrule and an optical fiber connector which is connected with the ferrule and in which an internal optical fiber is housed. The connection work is conducted by butting the end face of another optical fiber against the end face of the internal optical fiber (the housed fiber) in the optical fiber connector.
In the physical connecting method, a profile of the end face of optical fiber can seriously affect its connection characteristics. For example, when an angle (i.e., an angle to the optical fiber axis) of the end face of the optical fiber deviates from a right angle or the end face of the optical fiber is highly roughened, air may enter in a space between the end face of the optical fibers to butt and contact each other. As a result, Fresnel reflection increases at the end face of the optical fiber to cause an increase in connection loss of the optical fiber. A method for preventing the increase in connection loss of the optical fiber due to the roughness of the end face of the optical fiber is known that the end face of the optical fiber or the ferrule is polished after cutting the optical fiber.
Another method is known that the optical fibers are connected each other without polishing the end face of the optical fiber or the ferrule after cutting the optical fiber. This method is conducted such that a refractive index matching agent in liquid or grease state is interposed between the end faces of the optical fibers, where the agent has a refractive index similar to that of a core of the optical fiber (See JP-A-11-72641 and JP-A-11-101919). In this method, the refractive index matching agent is coated on the end face of the optical fibers, or filled into a connection portion of the optical fibers, and then the optical fibers are butted each other. Thus, air can be prevented from entering into the space between the end faces of the optical fibers, the Fresnel reflection caused by the air can be lowered, and the connection loss of the optical fiber can be reduced.
Another method is known that uses a refractive index matching film (See JP-B-2676705, JP-A-2001-324641 and JP-A-55-153912).
However, the method of polishing the end face of the optical fibers is not suitable in case of using a holey fiber as the external optical fiber or in case of conducting the connection work at the setting site of the optical fiber.
When the optical fibers are connected at the setting site, a lot of time consumption and labor charge are required for polishing the end face of the optical fibers etc., and a polishing device must be prepared at the setting site for polishing the end face of the optical fibers etc. Thus, the method is not suitable for connecting the optical fibers at low cost and simply.
Further, when the end face of the holey fiber is polished, a polishing residue generated during the polishing process and an abrasive agent used during the polishing process may enter the hole portion of the holey fiber to cause an increase in connection loss and a reduction in reliability.
On the other hand, the method of interposing the liquid or grease refractive index matching agent between the end faces of the optical fibers has the following problem. Since the refractive index matching agent generally is formed of a silicone or paraffin agent in liquid or grease state, it can enter into the hole portions of the holey fiber with time. Further, this method has the problem that, since the refractive index matching agent generally has a temperature dependency in refractive index, transmission loss of the holey fiber is extremely changed according to a change in refractive index of the refractive index matching agent entered into the hole portion of the holey fiber. Furthermore, this method has the problem that, when the refractive index matching agent enters into the hole portion of the holey fiber, the refractive index matching agent between the end faces of the optical fibers decreases so that air gaps and air bubbles are easily generated between the end faces of the optical fibers to cause the significant lowering of optical characteristics of the holey fiber.
As a measure against this problem, a method may be devised that the hole portions at the end face of the holey fiber are sealed before connecting the holey fiber. However, such a method is not suitable for connecting the optical fibers at the setting site since a dedicated connecting device is needed for performing the method and a lot of time consumption is required for sealing the hole portions at the end face of the holey fiber. Thus, the method is not suitable for connecting the optical fibers at low cost and simply.
Furthermore, the methods of using the film have the following problem. Since the diameter of the optical fiber is as fine as 80 or 125 μm, it is very hard to attach the film onto the end face of the optical fibers at high dimensional accuracy. Further, the film needs to have an adhesive ability or stickiness in order to attach the film onto the end face of the optical fibers. Therefore, when the optical fibers are connected each other at the setting site, a foreign substance such as a dust can easily adhere to the film to reduce reliability of the optical fiber and workability during the connection process of the optical fibers.
In addition, stress (strain) will apply to the resin used as the film when the optical fibers are connected. Since the refractive index of resin generally varies due to the stress applied, variation in transmission characteristics increases due to temperature change when the relaxation of stress is slow.
As an alternative method, fusion splicing may be used for the connection work.
However, since this method needs a fusion splicer, it is not suitable for an optical connector to be used at the setting site simply and for general purposes.
As described above, all the conventional connection methods have the problem in case of using the holey fiber as the external optical fiber or in case of conducting the connection work at the setting site of the optical fiber.